A CHANGE IN THE MIDPOINT POTENTIAL OF THE QUINONE Q(A) IN PHOTOSYSTEM-II ASSOCIATED WITH PHOTOACTIVATION OF OXYGEN EVOLUTION

The effect of photoactivation (the assembly of the Mn cluster involved in oxygen evolution) in Photosystem II (PS II)? on the redox midpoint potential of the primary quinone electron acceptor, Q(A), has been in vestigated. Measurements of the redox state of Q(A) were performed usi ng chlorophyll fluorescence. Cells of Scenedesmus obliquus were grown in the dark to obtain PS II lacking the oxygen-evolving complex. Growt h in the light leads to photoactivation. The midpoint potential of Q(A ) was shifted, upon photoactivation, from + 110 mV to - 80 mV. In cell s of a low-fluorescence mutant, LF1, that is unable to assemble the ox ygen-evolving complex but that has an otherwise normal PS II, the high er potential form of Q(A) was found. NH2OH treatment of spinach FS II, which releases the Mn and thus inactivates the oxygen-evolving comple x, causes an upshift of the redox potential of Q(A) (Krieger and Weis (1992) Photosynthetica 27, 89-98). Oxygen evolution can be reconstitut ed by incubation in the light in the presence of MnCl2, and CaCl2. Suc h photoactivation caused the midpoint potential of Q(A), to be shifted back from around +55 mV to lower potentials (- 80 mV), typical for ac tive PS II. The above results indicate that the state of the donor sid e of PS II has a direct influence on the properties of the acceptor si de. It is suggested that the change from the high- to the low-potentia l form of Q(A) may represent a mechanism for protection of PS II durin g the assembly of the O-2-evolving enzyme.